1. Field of the Invention
This invention is related, generally, to waterjet cutting systems, and, in particular, to a method and apparatus for controlling the orientation and position of a waterjet cutting head with respect to a surface.
2. Description of the Related Art
Waterjet and abrasive-jet cutting systems are used for cutting a wide variety of materials, including stone, glass, ceramics, and various metals, including stainless steel. Such systems are capable of cutting material thicknesses ranging up to and exceeding two inches. Thinner material may be stacked for cutting multiple pieces simultaneously.
In a typical fluid jet cutting system, a high-pressure fluid (e.g., water) flows through a cutting head having a cutting nozzle that directs a cutting jet onto a workpiece. The system may draw an abrasive into the high-pressure fluid jet to form an abrasive jet. The cutting nozzle may then be controllably moved across the workpiece to cut the workpiece as desired. After the fluid jet, or abrasive-fluid jet, generically referred to throughout as a cutting jet, passes through the workpiece, the energy of the cutting jet is dissipated and the fluid is collected in a catcher tank for disposal. Waterjet and abrasive-jet cutting systems of this type are shown and described, for example, in U.S. Pat. No. 5,643,058 issued to Erichsen et al., and assigned to Flow International Corporation of Kent, Wash., which patent is incorporated herein by reference, in its entirety. The '058 patent corresponds to Flow International's Paser 3 abrasive cutting systems.
FIG. 1 is an isometric view of a waterjet cutting system 100 in accordance with the prior art. The waterjet cutting system 100 includes a cutting head 120 coupled to a mount assembly 104. The mount assembly 104 is controllably driven by a control gantry (not shown in detail) having a drive assembly 108 that controllably positions the cutting head 120 throughout an X-Y plane that is substantially parallel to a surface 110 of a workpiece 112. Typically, the drive assembly 108 may include a pair of ball-screw drives oriented along the X and Y axes, each coupled to an electric drive motor. A Z-axis control mechanism 106 is coupled to the drive assembly and controls the position of the mount assembly in a Z-axis, substantially perpendicular to the surface 110.
Alternatively, the drive assembly 108 may include a five-axis motion system. Two-axis and five-axis control gantries are commercially-available as the WMC (Waterjet Machining Center) and the AF Series Waterjet cutting systems from Flow International of Kent, Wash.
The cutting head 120 includes a high-pressure fluid inlet 114 coupled to a high-pressure fluid source 116, such as a high-pressure or ultrahigh-pressure pump, by a high-pressure line 118. In this embodiment, the cutting head 120 includes a mixing tube 122 terminating in a jet exit port 124, from which a high-pressure stream of fluid, i.e., waterjet 126, is emitted and directed at the workpiece 112.
Although the term “mixing tube” is commonly used to refer to that portion of an abrasive-jet cutting system in which abrasive is mixed with a high-pressure fluid jet to form an abrasive cutting jet, in the following discussion, “mixing tube” may be used to refer to the nozzle through which a jet is discharged, regardless of whether the system uses an abrasive or non-abrasive cutting jet. In addition, the terms “waterjet” or “cutting jet” will be used to refer to the stream of fluid 126, also regardless of whether or not the stream includes abrasive.
A particular challenge in waterjet cutting systems is the provision of an appropriate support for the workpiece, inasmuch as any surface upon which the workpiece is supported will be subjected to the cutting force of the waterjet 126. A common system includes a grid 128 formed by a plurality of slats 130 positioned across a catcher tank (not shown). Upper edges of the slats 130 lie in a plane that is parallel to the X-Y plane. The workpiece 112 is supported on the grid 128 for cutting. A notch 134 (see FIG. 5) is cut into each slat 130 as the waterjet 126, penetrating through the workpiece 112, passes across the slat. The depth of the notch 134 will depend upon factors such as the traverse speed of the waterjet 126, and the thickness and hardness of the workpiece 112. The depth D of the slats 130, as shown in FIG. 1, is selected to tolerate significant exposure to the waterjet 126 as it repeatedly passes across the slat during successive cutting operations. Eventually, damage to the grid 128 reaches a level that the grid 128 must be replaced.
In operation, ultrahigh-pressure fluid is directed through an orifice (not shown) positioned in the cutting head to form an ultrahigh-pressure fluid jet 126. As discussed previously, the system may or may not entrain abrasive into the jet. The jet exits the mixing tube 122, whereby it is directed toward the workpiece 112. The cutting jet 126 pierces the workpiece 112 and performs the desired cutting. Using the control gantry, the cutting head 120 is traversed across the workpiece 112 in the desired direction or pattern.
To maximize the efficiency and quality of the cut, a standoff distance S (see FIG. 5) between the jet exit port 124 of the mixing tube 122 and the surface 110 of the workpiece 112 is controlled. If the standoff distance S is too small, the mixing tube 122 can plug during piercing, causing system shutdown and possibly a damaged workpiece 112. If the distance is too great, the quality and accuracy of the cut suffers. FIGS. 2 and 3 illustrate two known devices for determining the position of the workpiece relative to the mixing tube 122, for the purpose of establishing standoff D. The devices described with reference to FIGS. 2 and 3 are described in more detail in U.S. Patent Publication No. 2003/0037650 in the name of Knaupp et al. and assigned to Flow International Corporation of Kent, Wash., which publication is incorporated herein by reference, in its entirety.
The probe 138 of FIG. 2 is configured to extend, via actuation of a pneumatic cylinder, until it touches the surface 110 of the workpiece 112. The height of the surface 110 is thereby ascertained, the probe 138 is then withdrawn, the mixing tube 122 is positioned appropriately in the Z-axis, and cutting commences. FIG. 2 also shows a shield 136, configured to capture a significant amount of spray-back that occurs during a piercing operation, as described in more detail below.
The contact ring 140 of FIG. 3 is positioned coaxially with the mixing tube 122 and coupled to an actuator via a cantilevered rod 144. The contact ring 140 is configured to descend along the axis of the mixing tube 122 until it contacts the surface 110 of the workpiece 112. The height of the surface 110 having been established, the contact ring 140 may then be withdrawn or may be configured to remain in contact or near contact with the surface 110 during the cutting operation. Because a sensor associated with the contact ring 140 is capable of continuously monitoring the height of the surface 110, the associated cutting system can correct for changes in height of the workpiece 112. However, a device such as the shield 136 of FIG. 2 cannot be used concurrently with the contact ring 140.
When the system 100 is properly configured, and it cuts a continuous line through a workpiece 112, virtually all of the cutting fluid passes through the workpiece 112 to be captured in the catcher tank below. However, at the beginning of a cut while the waterjet 126 is impinging on a surface, but has not yet penetrated the surface, spray-back occurs, in which some or all of the fluid rebounds upward. Primary spray-back occurs while the waterjet 126 is first piercing the workpiece 112. In particular, a large portion of the primary spray-back occurs along an angle reciprocal to the angle of the waterjet 126, and thus, returns directly upward to the cutting device. This high-angle component of the spray-back also retains a significant fraction of the initial energy. Accordingly, it can be very damaging to components of the cutting system, especially in systems employing abrasives in the fluid stream.
FIG. 2 illustrates a spray-back shield 136 according to known art (described in more detail in the '650 publication). The shield 136 is configured to block and dampen the high-angle portion of spray-back and substantially prevents damage to components of the cutting system by the spray-back, and potential damage or injury to objects in the path of the spray-back.
As previously described, spray-back occurs when the waterjet 126 impinges but does not fully penetrate a surface. FIG. 4 illustrates a waterjet 126 traveling in direction T and cutting through a workpiece, and into slats 130 of the grid 128. The waterjet 126 loses energy as it passes through the workpiece, and cuts a notch 134 into the slats 130 to a depth N at which the energy of the waterjet 126 is insufficient to cut any deeper, although the energy remaining in the stream is still substantial. It may be seen that the advancing front 133 of the notch 134 has a curved shape as the waterjet 126 traverses the notch, while the bottom of each notch 134 is substantially horizontal.
For the purpose of this description, primary spray-back is that resulting from reflectance of the waterjet by a workpiece, while secondary spray-back results from reflectance of the waterjet by a structure beneath the workpiece.
Unlike the primary spray-back of a piercing operation, secondary spray-back, as illustrated in FIG. 4, is reflected back by the curved front 133 of the notch 134 in a fan shaped spray, in a direction substantially opposite the direction of travel T. The spray-back shield 136 captures only the highest-angle portion of the secondary spray-back. In some cases, depending on factors such as the speed, direction of travel T, the condition of the slat 130, the angle of the cut with respect to the slat 130, etc., a portion of the secondary spray-back can blast back through the kerf 132 of the workpiece 112 at a low angle and travel some distance from the cutting site. This kind of spray-back will be attenuated if the system is cutting a curved line, since the curved wall of the kerf will block some or all of the spray-back.
The most powerful secondary spray-back occurs when the direction of travel T is incident to the slat 130, that is, when the direction of travel T is parallel to the slat 130, and directly above, such that the waterjet 126 passes through the workpiece 112 and impinges directly on the upper surface of the slat 130 for an extended distance. In this configuration, very little of the cutting fluid can escape downward into the catcher tank, and so is driven upward through the kerf 132.
The mixing tube 122 is typically fabricated of specially formulated carbides to resist wear. Particularly for abrasive cutting systems, the mixing tube 122 suffers extreme wear due to its constant contact with high velocity abrasives. Thus, mixing tubes are a relatively expensive component of the system. The specially formulated carbides may also be brittle, and can easily break if the mixing tube 122 collides with an obstruction during operation of the cutting system 100, such as fixturing or cut-out portions of the workpiece 112 which may have been kicked up during the cutting operation. Accidental breakage of the mixing tube 122 increases operational costs and downtime of the cutting system 100.
Several collision sensor systems are known in the art. For example, a ring sensor, similar in appearance to the annular sensor 140 of FIG. 3, may be positioned in contact with, or just above the surface of a workpiece during a cutting operation. An obstruction will make contact with the ring portion of the sensor prior to contacting the mixing tube 122. The sensor is configured to respond to contact with the obstruction by initiating a shut down of at least the drive motors of the cutting system, and generally the waterjet 126 as well, to prevent damage to the mixing tube 122, and minimize damage to the workpiece.
Another collision detection system comprises a device having a portion of the cutting head configured to break away without damage to the mixing tube, in the event of a collision. The system is described in detail in U.S. Pat. No. 6,540,586, issued to Felice Sciulli et al., and assigned to Flow International Corporation of Kent, Wash., which patent is incorporated herein by reference, in its entirety.
Manipulating a jet in five axes may be useful for a variety of reasons, including, for example, cutting a three-dimensional shape. Such manipulation may also be desired to correct for cutting characteristics of the jet or for the characteristics of the cutting result. More particularly, as understood by one of ordinary skill in the art, a cut produced by a jet, such as the abrasive waterjet 126 of FIGS. 1-4, has characteristics that differ from cuts produced by more traditional machining processes.
Two of the cut characteristics that may result from use of a high-pressure fluid jet are referred to as “taper” and “trailback.” FIG. 5 shows an exemplary illustration of taper. The mixing tube 122 of FIG. 5 is traveling along an X-axis, perpendicular to the plane of the drawing. Taper refers to the angle A of a plane of one wall of the kerf 132 relative to a vertical plane. Taper typically results in a workpiece 112 that has different dimensions on the top surface 110 (where the jet 126 enters the workpiece) and the bottom surface 111 (where the jet 126 exits the workpiece).
FIG. 6 shows an example of trailback. The mixing tube 122 of FIG. 6 is traveling in direction T along the X-axis, parallel to the plane of the drawing. Trailback, also referred to as drag, is a condition in which the high-pressure fluid jet 126 exits the bottom surface 111 of the workpiece 112 at a point behind the point of entry of the jet 126 on the top surface 110 of the workpiece 112, relative to the direction of travel T. The trailback angle is the angle B of a line extending through a point of entry to a point of exit of the jet 126 relative to a vertical line.
These two cut characteristics, namely taper and trailback, may or may not be acceptable, given the desired end product. Taper and trailback vary, depending upon the thickness and hardness of the workpiece 112 and the speed of the cut. Thus, one known way to control excessive taper and/or trailback is to slow down the cutting speed of the system. Alternatively, in situations where it is desirable to minimize or eliminate taper and trailback while operating at higher cutting speeds, five-axis systems may be used to apply taper and lead angle corrections to the jet 126 as it moves along the cutting path, as illustrated in FIGS. 7 and 8.
It will be assumed, for the purpose of this description, that the portion of the workpiece to the right of the mixing tube 122 of FIG. 7 comprises the finished product, while the portion to the left is scrap. The mixing tube 122 is rotated around an axis parallel to the X-axis, until the right wall of the kerf 132 is substantially vertical.
As shown in FIG. 8, the mixing tube 122 is rotated around an axis parallel to the Y-axis, such that the waterjet 126 is angled into the direction of travel T until the trailback is substantially eliminated, as shown.
It will be recognized that, as the direction of travel T changes during the course of a cutting operation, the fourth and fifth axis rotations compensating for taper and trailback must change accordingly. A method and system for automated control of waterjet orientation parameters is described in U.S. Pat. No. 6,766,216 issued to Erichsen et al., and assigned to Flow International Corporation of Kent, Wash., which patent is incorporated herein by reference, in its entirety.